Method of forming a windage cover for a gas turbine engine the method including forming a continuous ring from a sheet of metal and bending and cutting the continuous ring to form at least two arcuate segments

ABSTRACT

A stator assembly having a windage cover for structure adjacent to a cavity bounded by rotating elements in a rotary machine is disclosed. Various construction details are developed for damping vibrations in the windage cover as the windage cover bounds a cavity having swirling high velocity gases that are capable of transmitting acoustic energy and kinetic energy to adjacent structure. In one detailed embodiment, the windage cover in the uninstalled condition has diverging arms that are resiliently compressed during installation to exert a frictional force on the adjacent structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 11/449,900 filedon Jun. 10, 2006, which issued as U.S. Pat. No. 7,635,251, and claimsthe benefit of the filing date thereof under 35 U.S.C. §120. Thisapplication incorporates by reference the specification including claimsand drawing of the parent application.

BACKGROUND OF THE INVENTION

This invention relates to stator assemblies of the type used in rotarymachines that have stator vanes, such as gas turbine engines. Moreparticularly, this invention relates to structure for aerodynamicallysmoothing surfaces that bound a cavity extending between a rotorassembly and a stator assembly.

Rotary machines are used to transfer energy between a flow path for astream of working medium gases and rotating elements inside the machine.There are many examples of such machines in widely disparate fields ofendeavor.

Axial flow gas turbine engines for industrial purposes and forpropelling aircraft are one example of such machines. These enginestypically have a compression section, a combustion section and a turbinesection disposed about an axis of rotation. An annular flow path forworking medium gases extends axially through the sections of the engine.The gases are compressed in the compression section. Fuel is burned withthe gases in the combustion section to add energy to the gases. Thepressurized, hot working medium gases are expanded through the turbinesection.

In the turbine section, the rotor assembly has a rotor disk and rotorblades that extend outwardly from the rotor disk. The rotor bladesextend across the flowpath for working medium gases. Each rotor bladehas an airfoil which adapts the rotor assembly to interact with theworking medium gases. The rotor blades receive work from the gasesflowing through the airfoils and drive the rotor assembly about the axisof rotation.

The rotor assembly transfers energy from the turbine section to thecompression section. In the compression section, the rotor assembly hasa rotor disk and rotor blades with airfoils that extend outwardly fromthe rotor disk. As the rotor assembly is driven about the axis ofrotation, the airfoils do work on the entering gases to compress thegases, increasing the concentration of oxygen in the gases for burningfuel with the gases in the combustion section.

The engine includes a stator assembly disposed about the rotor assembly.The stator assembly has an outer case to bound the flow path and arraysof stator vanes which extend inwardly across the working mediumflowpath. The arrays of stator vanes are disposed downstream andupstream of the adjacent arrays of rotor blades for guiding the gases toalign the incoming gases with the downstream array of rotor blades andto reduce swirl imparted to the gases by the upstream rotor blades. Thisis important because swirl represents wasted kinetic energy.

The stator assembly includes an inner shroud assembly which is supportedby the stator vanes. The shroud assembly includes a circumferentiallyextending seal land. The seal land is disposed radially about therotating structure to block the flow off gases between the statorassembly and the adjacent rotor assembly.

The shroud assembly and vane support structure for the shroud boundcircumferentially extending cavities that are inwardly of the flowpath.These cavities extend, for example, between the shroud assembly and theadjacent portions of the rotor assembly that carry the upstream anddownstream arrays of rotor blades. The shroud assembly has irregularprojections that extend into these cavities.

Working medium gases that leak from the flowpath fill these cavities.The rotor assembly bounding the cavity may rotate at ten thousand totwenty thousand revolutions per minute (10,000 20,000 rpm). As a result,the gases in the cavity are swept along by the boundary layer at therotor assembly, reaching mean wind velocities that may exceed fourhundred miles per hour (400 mph). (In comparison, the most severehurricanes have wind velocities of two hundred miles per hour (200 mph)which will cause storm surges in oceans of eighteen (18) feet and maycause catastrophic building failures.)

These winds are gases dragged along by the rotor assembly and constantlytake energy from the rotor assembly, and then lose this energy to thestator assembly by doing work on the adjacent structures throughfriction forces and by slamming into the irregular surfaces extendinginto the cavity. This type of work is typically called “paddle-wheelwork” or “stirring work.”. The energy is transformed from the usefulkinetic energy of rotation into heat, uselessly heating the gases andadjacent structures by several hundred degrees, and decreasing theefficiency of the engine. This may require the use of heavier and moreexpensive materials more forward in the engine than would otherwise berequired if the mass of such swirling gases could be reduced.

One solution is to provide aerodynamically smooth surfaces adjacent tothe high-speed wind cavities. These surfaces reduce the drag of thestator structure on the winds, and thus the need to constantly supplyenergy to the winds, by masking the winds from the irregular surfaces inthe cavity. However, the structures add weight to the engine, whichreduces engine efficiency. One possibility is to reduce the level ofadded weight by providing relatively lightweight structures that willprovide smooth aerodynamic contours to surfaces adjacent the windcavities. But then another problem arises because of the dose proximityof the rotor assembly to the stator assembly and to any structureprovided to stator assembly.

For example, as the rotor blades pass by each stator vane, each statorvane and the adjacent shroud structure experience pressure pulses fromeach passing rotor blade. As the rotor blades pass the stator structure,the stator structure is struck by a pressure rise from the passingpressure side of the rotor blade and experiences a pressure drop fromthe passing suction side of each rotor blade. A similar phenomenonoccurs as each rotor blade passes the suction side and pressure aside oneach stator vane. These pressure pulses take the form of significantacoustic energy which slams into the structure of the adjacent statorassembly and causes significant vibrations in these structures. As aresult, experience has shown that destructive vibrations can occur inthe adjacent structure and that such structures must be relativelystrong (with a concomitant increase in weight) to withstand the severewinds and acoustic energy adjacent these cavities.

Accordingly, scientists and engineers working under the direction ofApplicants' assignee have sought to develop relatively lightweightstructures that will provide aerodynamic smoothness to structures thatare adjacent to rotor-stator cavities and that have sufficientdurability to exist in that severe environment.

BRIEF SUMMARY OF THE INVENTION

This invention is in part predicated on the realization that relativelylightweight structures may be used to bound a rotor-stator cavityprovided that such structures provide significant damping to themselvesto avoid destructive vibrations in the structures. In one embodiment, itis predicated on recognizing that aerodynamically smooth structure mightemploy both coulomb friction damping and viscous damping if thestructure is thin enough to deflect in response to pressure pulses fromacoustic energy and from winds in the cavity.

According to the present invention, a circumferentially extendingannular member, such as a vane shroud, has a pair of damping surfacesspaced one from the other adjacent to a circumferentially extendingcavity in the shroud, the cavity having an upstream side and adownstream side, and further includes a windage cover for covering thecavity having at least two circumferentially extending segments of aring, each segment having a cross-section formed by a circumferentiallyextending base, the base having sides which extend circumferentiallyabout the base and two arms, each attached to one of the sides that aresubstantially perpendicular to the base, the arms being angled one tothe other in the uninstalled condition, and being deflected with respectto each other in the installed condition by engagement with the dampingsurfaces such that the arms press against the damping surfaces with aforce that causes coulomb damping at the damping surfaces.

In accordance with one embodiment of the present invention, the segmentof the windage cover has a channel-like (or U-shaped cross-sectionalshape) the arms diverging from one from the other in the uninstalledcondition such that they are compressed toward each other in theinstalled condition by engagement with the damping surfaces.

In accordance with one detailed embodiment of the present invention, thedamping surfaces are spaced radially and face each other, the baseextends circumferentially and radially to cover at least one side of thecavity, and the arms extend in a generally axial direction to engage theradially facing damping surfaces and position the covering base over thecavity.

In accordance with another embodiment of the present invention, thedamping surfaces are spaced axially and face each other, the arms extendin a generally radial direction to engage the axially facing dampingsurfaces to cover the upstream side of the cavity with one arm and thedownstream side of the cavity with the other arm, and the base extendscircumferentially and axially, to position the covering arms over thecavity.

According to the present invention, the method of forming the arcuatesegments of the ring includes forming a ring from a sheet of annularmaterial extending about and perpendicular to an axis Am, the sheethaving an inner diameter edge and an outer diameter edge; bending theinner diameter edge and a portion of the ring adjacent to the innerdiameter edge more parallel to the axis Am such that the ring has anannular inner flange at the inner diameter and a rim which extends fromthe flange to the outer diameter edge, the inner flange beingsubstantially perpendicular to the rim and parallel to the axis Am;cutting the ring to form at least two arcuate segments; wherein thestresses induced in the ring by the forming process cause each segmentto have a slightly greater radius of curvature than the ring and causesthe rim and the flange of the segment to be angled more with respect tothe axis Am, and diverge more away from each other as the arms extendfrom the ring than did the arms of the ring.

In accordance with one detailed embodiment of the method, the sheet ofmaterial for the method is relatively flat. The term “relatively flat”as used herein means that, prior to forming the flange, the elevationsof one part of the surface of the sheet with respect to other parts ofthe surface are less than twice the height of the flange.

In accordance with one detailed embodiment of the method, the methodincludes bending the outer diameter edge and a portion of the ringadjacent to the outer diameter edge more parallel to the axis Am suchthat the ring has an annular outer flange and the rim extends from theinner flange to the outer flange, the flanges being substantiallyperpendicular to the rim and parallel to the axis Am;

In accordance with one detailed embodiment of the method, the ring is afirst ring and prior to cutting the first ring, the method includesforming a second ring in the same fashion as the first ring, the secondring having a greater diameter than the first ring; and disposing therings about the axis Am such that the inner flange of the second ring isspaced radially from the inner flange of the first ring leaving anannular channel therebetween; forming the third and fourth rings in thesame fashion as the first and second rings with flanges adapted to facethe flanges on the first and second rings; disposing the third andfourth rings about the axis Am such that all rings have the same axis Amand the annular channel on the second ring axially faces the annularchannel on the first and second rings; disposing a circumferentiallyextending base member about the axis Am which engages the inner surfaceof the first flange of the first ring and the first flange of the thirdring; attaching the first and second rings together; attaching the thirdand fourth rings together; attaching the base member to the inner flangeof the first and third rings; cutting the first, second, third andfourth rings and the base along a plane containing the axis Am.

A primary feature of the present invention is a windage cover whichextends circumferentially about a stator assembly to reduce the size ofa circumferentially extending cavity bounded on one side by the statorstructure and which is adjacent to rotating structure. Another primaryfeature is the segmented nature of the windage cover. In one embodiment,each segment has a greater radius of curvature in the uninstalledcondition than in the installed condition. Another feature is thecross-sectional shape of the windage cover which has a base and two armswhich are deflected at installation to exert a damping force againstadjacent structure. In one embodiment, the base extendscircumferentially to cover a side of the cavity. In another embodiment,a pair of axially spaced arms extend from the base, and each of the armsextends radially and circumferentially to cover an associated side ofthe cavity. In one particular embodiment, a feature is seal landstructure which is attached to the base of the windage cover. In oneembodiment, a feature is using sheet metal to form the windage cover,the sheet metal being relatively thin compared to the area of thewindage cover and the radial height of the windage cover.

A feature of the method of forming the windage cover is the step ofattaching circumferentially continuous rings together to form acircumferentially continuous precursor to the windage cover prior tocutting the windage cover to form the segments of the windage cover.

A principal advantage of the present invention is the efficiency of therotary machine which results from reducing aerodynamic losses fromswirling gases by bounding a cavity in the stator structure with awindage cover. Still another advantage is the durability of the windagecover which results from decreasing vibrational stresses in the windagecover by damping vibrations in the windage cover. In one particularembodiment, the damping of vibrations results from using both coulombfriction and viscous friction to damp vibrations in the windage cover.In one detailed embodiment, damping by viscous friction results fromforming the windage cover of relatively thin sheet metal material whichpermits deflections of the walls of the windage cover in response tooperative forces from gases in the cavities adjacent to the windagecover. In one detailed embodiment, an advantage of the method is thecost and weight of the windage cover which results from using materialwhich may be bent, such as sheet metal material, to form the windagecover precursor and to form the windage cover precursor as a continuousring prior to cutting the precursor to form the segments of the windagecover. In one detailed embodiment, an advantage is the durability of therotary machine which results from trapping, with the windage cover,structure that might become loose in the cavity bounded by the windagecover. (An example of such structure is a nut and bolt fastener.) Anadvantage of the windage cover which is produced by the process ofattaching, continuous rings prior to cutting the precursor to thewindage cover is the efficiency of the rotary machine which results fromthe circumferential alignment of adjacent circumferential structure ofthe segments in the installed condition. In one particular embodiment,an advantage of the windage cover which is produced by the process isthe alignment of adjacent seal land segments in the installed conditionwhich are attached to the segments of the windage cover.

The foregoing features and advantages of the present invention willbecome more apparent in light of the following detailed description ofthe invention and the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a side elevation, cross sectional view of a rotary machine 10,such as a gas turbine engine, having a compression section 12 which ispartially broken away for clarity;

FIG. 2 is a side elevation, cross sectional view corresponding to theview in FIG. 1 and shows an alternate embodiment 184 of a segment of thewindage covers 84, 84 d shown in FIG. 1;

FIG. 2A is a simplified perspective view of a portion of the statorassembly shown in FIG. 2 showing the windage cover 184 and adjacentstructure of the stator assembly 18;

FIG. 2B is a simplified exploded, perspective view corresponding to theview shown in FIG. 2 during assembly of a segment of the windage cover184 u in the uninstalled condition to the shroud assembly 44;

FIG. 3 is a simplified cross-sectional view taken perpendicular to theaxis of rotation Ar with elements of the compression section broken awayfor clarity to shows four segments of the windage cover 184 and foursegments of the seal land 136 that are attached to the windage cover184.

FIGS. 4-12 relate to a method of forming the windage covers 84, 84 d,184.

FIG. 4 shows a front view of a sheet of annular material extending aboutand perpendicular to an axis Am that forms a ring precursor to anannular windage cover;

FIG. 5 is a cross-sectional view taken along a line 5-5 of FIG. 4 withphantom lines showing a portion of the sheet of material prior tobending the portion of the sheet;

FIG. 6 is a view corresponding to the view shown in FIG. 4 of a ring ofa windage cover and shows the effect of cutting the ring of the windagecover into segments.

FIG. 7 is a view corresponding to the view shown in FIG. 5 showing acontinuous ring after the step of cutting the continuous ring into foursegments and shows the effect of cutting the continuous ring.

FIG. 8 is a front view of an annular windage cover precursor 84 pextending about and perpendicular to an axis Am.

FIG. 9 is a cross-sectional view taken along a line 9-9 of FIG. 8 withthe phantom lines showing a portion of the sheet prior to bending theportion of the sheet;

FIG. 10 is a view corresponding to the view shown in FIG. 8 of thecompleted windage cover 84;

FIG. 11 is a view corresponding to the view shown in FIG. 9 and is takenalong the line 11-11 of FIG. 10 showing the completed windage coverafter the step of cutting the precursor windage cover into foursegments;

FIG. 12 is a partial perspective view of the windage cover precursor 184p for the windage cover 184 showing the precursor as a complete ringprior to cutting the precursor into four segments; the ring is sectionedand is partially broken away and shown without sectioning lines forclarity to show the attaching material.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows is a side elevation, cross sectional view of a rotarymachine 10, such as a gas turbine engine, having a compression section12. A portion of the compression section is shown in FIG. 1 and ispartially broken away for clarity. The engine includes an axis ofrotation Ar and an annular flowpath 14 for a stream of working mediumgases. The annular flowpath extends axially through components of thecompression section. These components include a rotor assembly 16 and astator assembly 18 which extend circumferentially about the axis ofrotation Ar.

The rotor assembly 16 includes a first rotor disk and blade assembly 22and a second rotor disk and blade assembly 22 d. Each rotor disk andblade assembly 22, 22 d has a rotor disk 24, 24 d and an array of rotorblades, as represented by the rotor blades 26, 26 d. The rotor bladesextend outwardly across the working medium flow path into proximity withthe stator assembly. The second rotor disk and blade assembly 22 d isspaced axially from the first rotor disk and blade assembly 22. Thisspacing leaves an annular opening 28 therebetween which is radiallyinwardly of the annular flowpath 14.

The rotor assembly 16 includes an annular seal member 32 which extendsaxially between the rotor disks 24, 24 d to inwardly bound the annularopening 28. The annular seal member includes circumferentially extendingprojections, as represented by the knife edges 34, which extend radiallyoutwardly into proximity with the stator assembly. The stator assembly18 includes an annular seal land, as represented by the segment of theseal land 36. The annular seal land extends circumferentially about andin close proximity to the outwardly extending knife edges 34.

The stator assembly includes an outer flowpath wall 38 (outer case). Theouter case outwardly bounds the annular flow path for working mediumgases 14. The stator assembly includes an array of stator vanes, asrepresented by the stator vane 42, and a shroud assembly 44 which issupported by the array of stator vanes. The array of stator vanes 42 andthe shroud assembly 44 extend inwardly from the outer case across theannular flowpath 14 into close proximity with the knife edges 34 of therotor assembly.

The array of stator vanes 42 and the shroud assembly 44 are disposeddownstream of the first rotor disk and blade assembly 22 and upstream ofthe second rotor disk and blade assembly 22 d. Each stator vane has anouter end 46 and an inner end 48. An airfoil 52 extends across theflowpath between the ends. The airfoil is located in a predeterminedmanner with respect to the adjacent rotor blades 26, 26 d for guidingthe working medium gases from the upstream rotor blades 26 to thedownstream rotor blades 26 d. In the embodiment shown, the airfoils arerotatable about a spanwise axis S to adjust the angle of the airfoil tothe approaching and departing stream of working medium gases. The shroudassembly includes a support 54 disposed therein for rotatably supportingthe inner end 48 of the stator vane about the spanwise axis S.

The shroud assembly 44 extends into the annular opening 28 andcircumferentially about the rotor assembly 16. The shroud assemblydivides the annular opening into three cavities: a first annular cavity56 extending between the shroud assembly and the first rotor disk andblade assembly 22; a second annular cavity 62 bounded by the shroudassembly; and, a third annular cavity 64 extending between the shroudassembly and the second rotor disk and blade assembly 22 d. The shroudassembly includes the circumferentially extending seal land 36 as partof the shroud assembly. The knife edges 34 of the rotor assembly 16 andthe seal land of the shroud assembly 18 cooperate to block the leakageof working medium gases from the working medium flowpath 14 through thefirst cavity 56 to the third cavity 64.

The shroud assembly 44 includes an outer annular member 66. An innerannular member 68 is spaced radially inwardly from outer annular member.The annular members radially bound the second cavity 62. The shroudassembly 44 has an upstream edge 70 and downstream edge 71. The support54 for the rotatable stator vanes 42 extends circumferentially in thesecond cavity between the inner annular member and the outer annularmember and is disposed between the edges. The support divides the secondcavity into an upstream portion 72 having an upstream side 74 and adownstream portion 76 having a downstream side 78. The support hasprojections that extend into the second cavity, as represented by thenut and bolt fasteners (not shown in FIG. 1). The nut and bolt fasteners79 are shown, for example, in FIG. 2A which is a perspective view, inpart, showing an alternate embodiment 184 of the windage cover 84. Asshown, the shroud assembly 44 has the inner and outer annular members68, 66 and the support 54 extending between the members which receivesthe nut and bolt fasteners 79. These components are the same type ofstructure as shown in FIG. 1 with slight differences in structure toaccommodate the different orientation and structure of the windage cover184.

Referring to FIG. 1, the outer and inner annular members 66, 68 havetracks which adapt the shroud assembly to receive one or more windagecovers. For example, the outer annular member 66 has a circumferentiallyextending track 80 adjacent the upstream side 74 of the second cavity 62and a circumferentially extending track 80 d adjacent the downstreamside 78 of the second cavity. Similarly, the inner annular member has acircumferentially extending track 82 adjacent the upstream side 74 ofthe second cavity and a circumferentially extending track 82 d adjacentthe downstream side 78 of the second cavity.

The shroud assembly 44 includes at least one segmented windage cover, asrepresented by a segment of the upstream windage cover 84 and by asegment of the downstream windage cover 84 d. Each windage cover extendscircumferentially about the shroud assembly. Each windage cover overlaysat least one side of the second cavity 62, such as the side 74 or theside 78. The windage covers shield projections, such as the nut and boltfasteners 79, that extend into the second cavity from the swirling gasesin the adjacent first cavity 56 or the adjacent third cavity 64. In thisparticular embodiment, the segmented windage covers are formed of atleast two circumferentially extending segments.

The downstream windage cover 84 d has structural elements that aresimilar to the upstream windage cover. The following description, whichapplies to the upstream windage cover 84, also applies to the downstreamwindage cover 84 d. The same numerical reference indicia are used forthose structural elements at or adjacent to the downstream windage coverthat are similar to structural elements at the upstream windage cover.In addition, the reference indicia for these elements at the downstreamwindage cover include the letter “d.” For example, the upstream windagecover has a base 88 which is similar to the base 88 d of the downstreamwindage cover. FIG. 1 shows the windage cover 84 in the installedcondition in full and, in phantom, shows the windage cover Mu in theuninstalled condition. FIG. 11 also shows the windage coverage in theuninstalled condition Mu.

Referring to FIG. 1 and FIG. 11, each segment of the upstream windagecover 84 has a U-shaped cross-sectional shape which is formed by thebase 88 and two arms 96, 98. The base 88 extends circumferentially andradially to cover at least one of the sides of the second cavity 62, asrepresented by the side 74. The base has a first side, as represented bythe outer side 90, and a second side, as represented by the inner side92, which extend circumferentially about the base. The inner side isspaced from the outer side leaving a base member 94 which extendstherebetween. In the present embodiment, the base and the arms of eachsegment of the windage cover are formed as one piece. An alternateembodiment might be formed of a segment of a first ring having anL-shaped cross-section for one arm and a segment of a second ring havingthe mirror image of an L-shaped cross-section for the second arm. Suchan embodiment might have the arms joined, for example, at the bottom ofthe “L” to a base member having the form of an annular plate with theupright of the “L” shaped cross-section extending to form the arm.

The two arms 96, 98 of each segment of the upstream windage cover 84include the first arm, as represented by the outer arm 96 and the secondarm, as represented by the inner arm 98. As shown in FIG. 1, the outerarm and the inner arm in the installed condition are substantiallyperpendicular to the base 88 of the windage cover. Each arm extends froman associated side 90, 92 of the base in a generally axial direction toengage the annular members 66, 68 of the stator assembly and to positionthe base 88 over the second cavity. As shown in FIG. 11, the outer arm96 of the windage cover has a first cover surface 102 which engages theouter annular member 66. The inner arm 98 has a second cover surface 104which engages the inner annular member 68. In a like manner as shown inFIG. 1, the downstream segment of the windage cover 84 d has the sameelements denoted by the letter “d.”

The outer and inner annular members 66, 68 of the stator assembly havedamping surfaces 106,108 which adapt the shroud assembly to engage theradially facing cover surfaces 102,104 of the arms 96, 98. For example,the first damping surface 106 and the second damping surface 108 form apair of damping surfaces adjacent to the upstream side 74 of thecircumferentially extending second cavity 62. The first and seconddamping surfaces are spaced one from the other and face each other. Thefirst damping surface 106 faces radially inwardly and extendscircumferentially about the outer annular member 66. The second dampingsurface 108 faces radially outwardly and extends circumferentially aboutthe inner annular member 68. In a like manner, the stator assembly 18has a downstream first damping surface 106 d and a downstream seconddamping surface 108 d that adapt the shroud assembly to receive thedownstream stream windage cover 84 d. Thus, the downstream dampingsurfaces form a second pair of damping surfaces that are spaced one fromthe other and that face each other.

The uninstalled condition of the segment of the upstream windage cover84 u is shown in phantom in FIG. 1. In the uninstalled condition, thearms 96, 98 are spaced one from the other and angled one to the othersuch that the arms diverge as the arms extend in the axial direction. Inthe uninstalled condition, the first cover surface 102 is spaced fromthe second cover surface 104 by a maximum distance D. In the installedcondition 84, the arms of the windage cover are deflected with respectto each other by engagement with the damping surfaces 106,108 such thatthe installed maximum distance D′ between said cover surfaces isdifferent from and, in this case smaller than, the uninstalled distanceD between the cover surfaces in the uninstalled condition. As a result,the cover surfaces of the arms press against the damping surfaces with aforce that causes coulomb damping under operative conditions at thedamping surfaces.

The windage covers 84, 84 d are also fabricated such that the coversurface 102, 104 of each arm 96. 98 has a larger radius of curvature inthe uninstalled condition than does the damping surface 106, 108 that isengaged by the associated cover surface. (The radii are measured at thelocation on the damping surface 106 or the cover surface 102 that isdosest to the adjacent side 90 of the base 88 where the cover surfacefirst engages the damping surface; or dosest to the adjacent side 92 forthe damping surface 108 or the cover surface 104). As a result, underoperative conditions the differences in the radii of curvature betweenthe damping surfaces and the arms in the uninstalled condition causes anincrease in frictional forces between the damping surface and the coversurface and results in additional coulomb damping. For example, thefirst damping surface 106 of the outer annular member 66 has a radius ofcurvature Ro. The outer cover surface 102 on the outer arm 96 (firstarm) in the installed condition has a radius of curvature Roai at theupstream side 90 (first side) of the base 88 that is equal to the radiusof curvature Ro. However, the outer cover surface 102 in the uninstalledcondition of the windage cover Mu has a radius of curvature Roau at thefirst side of the base that is a greater than the radius of curvatureRoai.

Similarly, the inner damping surface 108 (second damping surface) on theinner annular member 68 has a radius of curvature Ri. The inner coversurface 102 (second cover surface) on the inner arm 98 in the installedcondition has a radius of curvature Riai that is equal to the radius ofcurvature Ri at the second side 92 of the base 88. However, the innercover surface 104 of the windage cover 84 u in the uninstalled conditionhas a radius of curvature Riau at the second side of the base that isgreater than the radius of curvature Riai.

FIG. 2 is a simplified side elevation, cross sectional view of a segmentof the windage cover 184 which is shown in more detail in FIG. 2A and isan alternate embodiment of the windage covers 84, 84 d shown in FIG. 1.The numerical reference indicia for elements of the windage cover 184and selected elements of the shroud assembly that are engaged by thewindage cover have numerical reference indicia that are increased by 100from the reference indicia used in FIG. 1. Many elements of thecompression section have the same reference indicia as FIG. 1 becausethe differences between embodiments lie in the design of the windagecover and in the different way of positioning the windage cover withrespect to the remaining elements of the shroud assembly. Thus, therotor assembly in FIG. 2 is the rotor assembly 16, the stator assemblyin FIG. 2 is the stator assembly 18, and the windage cover in FIG. 2 isthe windage cover 184 having arms 196,198.

FIG. 2A is a perspective view of a portion of the stator assembly shownin FIG. 2 showing the circumferential ends of the outer annular member66 and the inner annular member 68 and the windage cover 184 as thewindage cover is being slid into position during assembly. FIG. 2A alsoshows adjacent structure of the stator assembly such as the stator vanes42 and other structure of the shroud assembly 44. As shown in this view,the support 54 includes projections that extend into the second cavity,as represented by the nut and bolt fasteners 79, which are not shown inthe view shown in FIG. 2.

FIG. 2B is a simplified exploded, perspective view corresponding to theview shown in FIG. 2A during assembly of a segment of the uninstalledwindage cover 184 u to the shroud assembly 44. The windage cover isshown in full with axial forces being exerted on the walls 196,198during installation to align the walls with tracks, as represented bythe grooves 180, 180 b, prior to circumferentially inserting the windagecover segment into the tracks. FIG. 2B also shows, in phantom, portionsof the walls 196, 198 of the segment of the windage cover free ofexternal forces in the uninstalled condition. Thus, FIG. 2B also showsin exaggerated fashion the diverging arms 196, 198 of the windage coverin the uninstalled condition 184 u. The windage cover has a distance Dwhich extends between the arms and a distance D′ which extends betweenthe arms in the installed condition. Assembly is accomplished bycircumferentially sliding the windage cover segment 184 u into tracks180, 180 d at the outer annular member 66 and tracks 182, 182 d at theinner annular member 68 while slightly deflecting the arms 196, 198toward each other.

Referring to FIG. 2, FIG. 2A, and FIG. 2B, the stator assembly 18includes an annular seal land, as represented by the segment of the sealland 136. The annular seal land extends circumferentially about and inclose proximity to the outwardly extending knife edges 34 of the rotorassembly. Unlike the seal land 36 of FIG. 1 which is attached directlyto the inner annular member 68, the seal land 136 is attached to thewindage cover 184 and, is in turn, supported and positioned by the innerannular member 68.

As noted above with respect to the windage covers 84, 84 d, and as shownin FIG. 2A and FIG. 2B, for example, the outer annular member 66 and theinner annular member 68 have tracks, as represented by the grooves 180,180 d. The groove 180 is adjacent to the upstream side 74 of the secondcavity 62. The circumferentially extending groove 180 d is adjacent thedownstream side 78 of the second cavity. Similarly, the inner annularmember 68 has a circumferentially extending upstream track 182 adjacentthe upstream side 74 of the second cavity and a circumferentiallyextending track 182 d adjacent the downstream side 78 of the secondcavity.

The windage cover 184 extends circumferentially about the shroudassembly 44. The windage cover has an upstream side 186 that overlaysthe upstream side 74 of the second cavity 62, and a downstream side 186d that overlays its downstream side 78. The windage cover shieldsprojections that extend into the second cavity from the swirling gasesin the adjacent first cavity 56 or the swirling gases in the adjacentthird cavity 64 that are shown in FIG. 2. The projections arerepresented by the projections 79 in FIG. 2A. The segmented windagecover 184 may be formed of at least two circumferentially extendingsegments. As shown in FIG. 3 for this particular embodiment, the windagecover is formed of four circumferentially extending segments. Similarly,the inner annular support member 68 may be formed of at least twosegments and, as shown, the outer annular support member 66 is formed offour segments.

As shown in FIG. 2, FIG. 2A, and FIG. 2B, the windage cover 184 has aU-shaped cross-sectional shape which is formed by the base 188, thefirst, upstream arm 196 having the upstream side 186 of the windagecover; and, the second, downstream arm 198 having the downstream side186 d of the windage cover. The base 188 is a segmented ring whichextends circumferentially and which engages the inner annular member 68.The base has a first side, as represented by the upstream side 190, anda second side, as represented by the downstream side 192, which extendcircumferentially about the base. The sides 190, 192 are closelyadjacent to the sides 186, 186 d of the windage cover. Each arm extendsfrom the associated side 190, 192 of the base in a generally radialdirection. The upstream arm 196 and the downstream arm 198 in theinstalled condition are substantially perpendicular to the base 188 ofthe windage cover 184.

As shown in FIG. 2B, the upstream side 190 of the base 188 is spacedaxially from the downstream side 192 leaving a base member 194 which isdisposed therebetween. In the present embodiment, the base of thewindage cover includes the bottom flanges of two rings: an upstream ring302 having an L-shaped cross-section formed by an upright 304 and ashorter bottom flange 306; and, a downstream ring 302 d having across-section which is the mirror image of an L-shaped cross-section.The cross-section of the downstream ring is formed by an upright 304 dand a shorter bottom flange 306 d.

The base is formed by attaching the base member 194 to the bottomupstream flange 306 of the upstream ring 302 and the bottom downstreamflange 306 d of the downstream ring 302 d. An alternate embodiment mightbe formed of one piece by using a cylindrical base member 194coextensive with the base 188 and handing additional axial width. Thearms of the windage cover arm are formed by bending the additionalmaterial away from the base until the arms extend radially. Such aconstruction may be formed, for example, by a rolling mill.

As discussed in more detail with respect to FIG. 12, the rings 302, 302d cooperate with a pair of associated rings 312, 312 d. The associatedrings 312,312 d have a larger inner diameter and cooperate to form thearms and retention flanges. The rings 312, 312 d have matching retentionflanges 316, 316 d spaced radially from the base flanges 306, 306 dleaving annular channels 317, 317 d therebetween. The channels adapt thewindage cover to engage the inner annular member 68. The arm 196 isformed with an upright 314 cooperating with the upright 304 and the arm198 is formed with an upright 314 d cooperating with upright 304 d.Thus, in the assembled condition, the first base flange 306 and secondarm flange 316 extend from the upstream arm in the axial, downstreamdirection. The third and fourth flanges 306 d, 316 d extend from thedownstream arm in the axial, upstream direction and face the first andsecond flanges.

Each arm is positioned over the second cavity by the base 188 and by thearm flanges 316, 316 d engaging the inner annular member. Each arm alsoengages the outer annular members 66 of the stator assembly 18. Inparticular, the upstream arm 196 of the windage cover has a first,upstream outer cover surface 202 which engages the outer annular member66. Similarly, the downstream arm 198 has a second, downstream outercover surface 204 which also engages the outer annular member 66.

As mentioned, the FIG. 2 embodiment also has windage cover surfaces thatengage the inner annular member 68. The inner annular member 68 isengaged by cover surfaces on the base flanges 306, 306 d and by coversurfaces on the second and fourth arm flanges 316, 316 d. In particular,the first upstream flange 306 has a third, upstream base cover surface308 and the third, downstream stream flange 306 d has a third,downstream base cover surface 308 d. The second, upstream arm flange 316has a fourth, upstream cover surface 318 and the fourth, downstreamstream arm flange 316 d has a fourth, downstream cover surface 318 d.

As shown in FIG. 2A and FIG. 2B, the outer annular member 66 and innerannular member 68 of the stator assembly 18 have damping surfaces thatadapt the annular members to engage the windage cover surfaces. A firstdamping surface 206 faces axially downstream and the second dampingsurface 206 d faces axially upstream toward the first damping surface.The damping surface 206 engages the radially extending cover surface 202on arm 196, and the damping surface 206 d engages the radially extendingcover surface 204 on the arm 198. The cover surfaces and dampingsurfaces are substantially parallel in the installed condition. Thefirst damping surface 206 is disposed adjacent to the upstream side 74of the circumferentially extending second cavity 62. The second dampingsurface 206 d is disposed adjacent to the downstream side 78 of thecircumferentially extending second cavity 62. Accordingly, the first andsecond damping surfaces are a pair of damping surfaces that are spacedone from the other and that extend circumferentially about the outerannular member 66.

The shroud assembly 44 has other damping surfaces that face radially forengaging associated cover surfaces. For example, the annular innermember 68 has third upstream and downstream base damping surfaces 212,212 d which face inwardly. The base damping surfaces 212, 212 d engagethe windage cover surfaces 308, 308 d on the base flanges 306,306 d. Theannular inner member 68 has fourth upstream and downstream flangedamping surfaces 208, 208 d that face outwardly and engage the windagecover surfaces 318, 318 d on the axial arm flanges 316, 316 d.

As mentioned earlier, FIG. 2B shows in full the segment of the windagecover 184 during the installation condition with the arms compressed toalignment in the arms with the grooves 180, 180 d. FIG. 2B also shows inphantom the segment of the upstream windage cover 184 u in theuninstalled condition free of external forces. In the uninstalledcondition, the arms 196, 198 are spaced one from the other and angledone to the other such that the arms diverge as the arms extend in theradial direction. In the uninstalled condition, the first cover surface202 is axially spaced from the second cover surface 204 by a maximumdistance D. In the installed condition 184, the arms of the windagecover are deflected with respect to each other by engagement with thedamping surfaces 206, 206 d such that the installed maximum distance D′between said cover surfaces is different from and, in this case, smallerthan the uninstalled distance D between the cover surfaces in theuninstalled condition. As a result, the cover surfaces of the arms pressagainst the damping surfaces with a force that causes coulomb damping atthe damping surfaces. As will be realized an alternate embodiment mighthave the arms inclined toward each other, with the first and seconddamping surfaces being the adjacent surfaces of the grooves 180, 180 dfacing axially, but facing axially away from each other with thedistance D in the uninstalled condition being smaller than the distanceD′ in the installed condition.

The windage cover 184 is designed in a fashion similar to the windagecovers 84, 84 d to cause a slight mismatch in radii of curvature betweenradially facing cover surfaces of the windage cover and the associateddamping surfaces on the annular members 66, 68. For example, the thirdcover surfaces 308, 308 d of the base have a larger radius of curvaturein the uninstalled condition than does the inner surface of the annularmember 68 (third damping surfaces 212, 212 d) that is engaged by thebase. As a result, under operative conditions the differences in theradii of curvature between the damping surfaces and the arms in theuninstalled condition causes an increase in frictional forces betweenthe damping surface and the cover surface and results in additionalcoulomb damping. As shown in FIG. 2 by way of illustration, the thirddamping surface 312, 312 d of the inner annular member 68 has a radiusof curvature Rs. In the installed condition, the third cover surfaces308, 308 d on the base have radii of curvature Rbi that are nearly equalto the radius of curvature Rs. However, as shown in FIG. 12, the thirdcover surfaces 308, 308 d of the base in the uninstalled condition ofthe windage cover 184 u have radii of curvature Rbu that are greaterthan the radius of curvature Rbi on the base in the installed conditionand the radius of curvature Rs of the third damping surfaces on theinner annular member 68.

FIG. 3 is a simplified cross-sectional view taken perpendicular to theaxis of rotation Ar with elements of the compression section broken awayfor clarity. FIG. 3 shows the four segments of the windage cover and thefour segments of the seal land 136, that are attached to the windagecover 184. As discussed earlier, the windage cover has an upstream arm196 which engage the outer annular member 66 of the shroud assembly 44.

The windage covers 84, 84 d and 184 bounding said at least one side ofthe second cavity 62 and facing, for example, the first cavity 56 or thethird cavity 64 are each formed of sheet metal having at least one layerof sheet metal and having a total thickness of all layers that is lessthan about ninety mils (ninety thousandths of an inch or abouttwenty-three hundred micrometers). The material for the layers of thewindage cover are typically alloys. One particular family of alloysfound satisfactory are nickel based super alloys such as the Inconel®family of materials provided by the Special Metals Corporation. Oneparticular alloy known to be suitable is described as Aerospace MaterialSpecification (AMS) 5599 material. An example of such material isInconel® 625 material. Other suitable materials are AMS 4919(Ti-6-2-4-2) and AMS 4911 (Ti-6-4). The material may be worked, forexample, by hot forming or cold forming using rolling mills.

As discussed earlier, a particular advantage is the relativelylightweight of the windage cover while still providing adequate dampingto itself through coulomb friction. In addition, the ratio of therelative thickness of the windage cover to its radial height and inother embodiments to its radial height and area enables viscous frictionto dissipate vibrations induced by pressure pulses of the adjacentcavities. This results because the pressure pulses of the adjacentcavities causing slight deflections of portions of the windage coverstructure bounding the second cavity and extending to bound the adjacentcavities. The vibrations of the windage cover adjacent the cavities, inturn, act upon gases in the second cavity and the adjacent cavities.Thus, the design uses the vibrations themselves to cause vibrationalenergy to be just dissipated by fluid friction. This results from theadjacent gases in the second cavity resisting the forces of the gases inthe first and third cavities by a force proportional to the velocity ofthe deflections of the windage cover and in a direction opposite to thedirection of movement of the windage cover. In the particularembodiments shown in FIG. 1, the windage cover has a radial height R forthe upright portion of the windage cover, such as for the uprights304,314, bounding said at least one side of the second cavity. Thewindage cover has an average axial thickness T for the portion boundingboth the second cavity and the adjacent cavity. The ratio R/T is greaterthan eighteen (R/T>18) such that pressure pulses from the gases in theadjacent first cavity cause the windage cover to move against the gasesin the second cavity and through fluid friction, to viscously dampvibrations in the windage cover.

Portions of the windage cover bounding the cavities shown are in theform a sheet-like structure when comparing the relative thickness of theportion to its area. For example, in the embodiments shown in FIG. 1 andFIG. 2, the windage cover might have a ratio of radial height R tothickness T that is greater than eighteen (R/T>18); and also might havean area A facing the second cavity for the portion having the radialheight R, a ratio of area A to the thickness T that is greater thanfifteen hundred (A/T>1500). Experimental testing has shown that such athin, flat sheet will accommodate the vibrations that are induced in thestructure under operative conditions.

There are several satisfactory methods for forming a windage cover. Onemethod of forming the segments might include the step of cuttingprecursor segments of the windage cover from a relatively flat, piece ofmaterial, and then bending the edge or edges of the segments to form thestructure having the cover surfaces 106, 108 on windage cover 84 or thecover surfaces 206, 206 d on the windage cover 184. The bending of thearms, such as arms 96, 98, 196, 198, to cause the arms to diverge mighttake place after forming the segments. In the method of forming thesegments of the windage cover 184, this would require assembling andattaching the individual parts of each segment of the rings togetherfour times to form each of the four segments in completed form for thewindage cover 184.

FIGS. 4-12 relate to another method of forming the windage covers 84, 84d, 184 that has advantages over the method discussed above for formingthe windage cover. In particular, the methods for forming the windagecover 84, 184 discussed below include the following two groups ofsequential steps, that is: a first group which are the steps for formingas a complete ring, a ring precursor having parts of the windage coverthat engage the annular members 66, 68 of the shroud assembly 44. Thering precursor has these parts in position with respect to each otherand attached together. This group of steps is performed prior toperforming the second group of steps. The second group of steps arethose that cut the precursor rings of the windage covers 84 p, 84 dp,184 p into segments to form the segments of the windage cover. In onedetailed method of forming the windage cover 184, the steps includeattaching all of the rings together that have parts that engage theannular members 66, 68 prior to cutting the rings into segments.

As noted above, each segment of the windage covers 84, 84 d, 184 has across-sectional shape which includes a circumferentially extending base88, 188. Each base has a first side 190, 192 which extendscircumferentially about the base and at least one arm, 196, 198extending from the first side which is substantially perpendicular tothe base in the installed condition. For example, windage cover 84 hasarms 96, 98; and, windage cover 184 has arms 196, 198.

FIG. 4-7 shows by the use of a single ring precursor 322 p an importantphysical aspect of a segment of a windage cover that results fromforming a windage cover with the two groups of steps. In particular,FIG. 4 shows a front view of a sheet of annular material extending aboutand perpendicular to an axis Am, as represented by the ring precursor322 p for part of an annular windage cover 322. FIG. 5 is across-sectional view taken along the line 5-5 of FIG. 4 with the phantomlines showing a portion 328 of the sheet prior to bending that portionof the sheet. Prior to bending a portion of the sheet, the ring has aninner diameter edge 324 and an outer diameter edge 326. The portion ofthe sheet is bent to form a flange 328 leaving the remainder as a rim332.

FIG. 6 is a view corresponding to the view shown in FIG. 4; and, FIG. 7is a view corresponding to the view shown in FIG. 5 showing thecontinuous ring precursor 322 p after the step of cutting the continuousring into four segments to form the ring 322 for part of a windagecover. The stresses induced in the ring precursor 322 p by the formingprocess cause each segment 322 to have a slightly greater radius ofcurvature Ru than the radius of curvature Rp of the ring precursor andcauses the rim 332 and the flange 328 of the segment to be angled morewith respect to the axis Am. As a result, the structure diverges fromthe location the structure will have in the installed condition.

The method of forming the windage cover includes steps comprising:forming a continuous ring from a sheet of annular material extendingabout and perpendicular to an axis Am, the sheet having an innerdiameter edge 324 (as shown in full and by the phantom lines in FIG. 5)and an outer diameter edge 326. The sheet is relatively flat. Thecompletion of the step of bending the inner diameter edge and a portionof the continuous ring adjacent to the inner diameter edge more parallelto the axis Am is shown in FIG. 5. The bending might be done in arolling mill and is done such that the ring has an annular inner flange328 at the inner diameter and a rim 332 which extends from the flange tothe outer diameter edge 326. The inner flange is formed substantiallyperpendicular to the rim and parallel to the axis Am. The method alsoincludes the step of cutting the ring to form at least two arcuatesegments, and as shown, forms four arcuate segments.

As noted above, the sheet used to form the continuous ring is relativelyflat. The term “relatively flat” as used herein means that, prior toforming the flange, the elevation of one part of the surface of thesheet with respect to other parts of the surface is less than twice theheight of the largest flange that is formed by the step of bending thematerial.

FIG. 8 shows a front view of an annular windage cover precursor 84 pextending about and perpendicular to an axis Am. FIG. 9 is across-sectional view taken along a line 9-9 of FIG. 8 with the phantomlines showing a portion of the sheet prior to bending the portion of thesheet. FIG. 10 is a view corresponding to the view shown in FIG. 8 ofthe completed windage cover 84 and FIG. 11 is a view corresponding tothe view shown in FIG. 9, with FIG. 10 and FIG. 11 both showing thecompleted windage cover after the step of cutting the precursor windagecover into four segments. The windage cover has a base 88 having a firstside 90 and a second side 92, a first arm 96 extending from the firstside and a second arm 98 extending from the second side.

The step of forming the inner flange 328 of the ring precursor 322 p asshown in FIG. 4 and FIG. 5 adapts the method to form a first flangecorresponding to the arm 98 which extends from a side of the base and touse the first flange to form the arm 98 in the finished condition of thewindage cover 84. The step of forming at least a portion of the rim 332of the ring precursor 322 adapts the method to form at least a portionof the base with the base extending between the sides in the finishedcondition of the windage cover 84. The method may also include bendingthe rim to form the frustoconical section of the base or the relativelyflat ring may have the frustoconical section already formed.

Prior to the step of cutting the ring, the method of forming the windagecover 84 also includes the step of bending the outer diameter edge and aportion of the ring 84 p adjacent to the outer diameter edge as wasshown for the inner diameter edge of the ring precursor 322 in FIG. 4and FIG. 5. This portion of the rim is bent more parallel to the axis Amsuch that the ring has an annular outer flange or arm 96 and the rimextends from the inner flange to the outer flange to form the base 88,with the flanges (arms 96, 98) being substantially perpendicular to therim and parallel to the axis Am. The annular outer flange adapts thering to form the outer arm 96 p of the windage cover precursor 84 pleaving the base precursor 88 p therebetween. The windage coverprecursor 84 p extends about the axis Am and has a radius Roap to theouter arm 96 and a radius of Riap to the inner arm 98. The radius Roapto the outer arm 96 is equal to the radius Ro to the first dampingsurface and the radius Riap to the inner arm 98 is equal to the radiusto the second damping surface Ri. Thus, the windage cover precursor 84 pis formed to the radii Roai and Riai that it will have in the installedcondition and are equal respectively to the radii Ro, Ri of the dampingsurfaces.

Upon cutting the windage cover precursor 84 p into segments, thestresses induced in the ring (windage cover precursor 84 p) by theforming process, cause each segment to have the slightly greater radiiof curvature Roau and Riau of the uninstalled condition Mu which aregreater than the radii of the windage cover precursor 84 p. In addition,the stresses cause the arms of the segment to be angled more withrespect to the axis Am as was shown in FIG. 7 for the ring 322. This inturn causes the arms to diverge from the location the structure willhave in the installed condition. As a result, the arms of the segment ofthe windage cover diverge more away from each other as the arms extendfrom the base than do the same arms prior to the step of cutting thering. This causes the distance D between the arms in the uninstalledcondition to be greater than the distance D′ which is the distancebetween the arms in the installed condition discussed above.

FIG. 12 is a partial perspective view partially broken away andsectioned, of the windage cover precursor 184 p for the windage cover184 prior to cutting the precursor into four segments. The windage coverprecursor has the same radii to windage cover surfaces (which engage theinner and outer annular members 66, 68 for damping) that the coversurfaces will have in the installed condition. The windage coverprecursor 184 p is cut by a sectioning plane containing the axis Am.shown in FIG. 2 and FIG. 2B. FIG. 2B shows the segment of the windagecover 184 u in the uninstalled condition after being segmented. In thisview, the segment of the windage cover is being inserted into thegrooves 180,180 d. This requires pressing the arms toward each other toensure the distance between the arms is the distance D′ as the segmentof the windage cover is installed in the grooves.

The windage cover precursor 184 p is a circumferentially extending ringformed of the four rings 302,302 d,312,312 d discussed above. Each ofthe rings is formed as a circumferentially continuous ring using themethod shown in FIG. 4 and FIG. 5. As noted above, the perspective viewin FIG. 12 is partially broken away with section lines eliminated toshow more clearly the relationship of the rings to each other and to thematerial used to attach the rings together, such as by brazing using asuitable braze material.

The method of forming the windage cover precursor 184 uses the ring 302and the method for forming the ring is shown in and discussed withrespect to FIG. 4 and FIG. 5. As shown in FIG. 12, the first ring 302has the first flange 306 having the axially oriented outer surface 308and an axially oriented inner surface 309. The method also includes thesteps of forming the second ring 312 having a second flange 316 in thesame fashion as the flange on the first ring. The second flange of thesecond ring has a greater diameter than the first flange of the firstring. The method includes disposing the rings about the axis Am suchthat the second flange of the second ring is spaced radially from thefirst flange of the first ring leaving the annular channel 317therebetween. The steps include forming a third ring 302 d in the samefashion as the first ring, the third ring having a third flange 306 dwhich has an axially oriented inner surface 309 d; and forming a fourthring 312 d in the same fashion as the second ring 312, the fourth ringhaving a fourth flange 316 d. The third and fourth flanges 306 d, 316 dare oriented to face the first and second flanges 306, 316 on the firstand second rings in the installed condition. Thus, the steps includedisposing the third and fourth rings 302 d, 312 d about the axis Am suchthat all rings have the same axis Am and the third and fourth flanges306 d, 316 form an annular channel 317 d for the third and fourth ringsthat axially faces the annular channel 317 of the first and secondrings.

Further steps include disposing a circumferentially extending basemember 194 about the axis Am. The base member engages the inner surface309 of the first flange 306 of the first ring and the inner surface 309d of the third flange 316 of the third ring 302 d; attaching the firstand second rings 302, 312 together; attaching the third and fourth rings302 d,312 d together; attaching the base member 194 to the inner flange306 of the first ring and the inner flange 306 d of the third ring toform the base 188. The preceding steps may be performed in any order andthe base member need not be a circumferentially continuous ring. Aftercompleting the foregoing steps, the method includes cutting the first,second, third and fourth rings and the base along a plane containing theaxis Am.

As noted above, brazing is one satisfactory method for attaching therings together, and for attaching the rings and base member. The methodmight also include attaching the seal land to the rings prior to cuttingthe rings. The radius of curvature Rbp of the third cover surfaces ofthe base in the precursor 184 p is equal to the radius of curvature ofthe third damping surfaces of the inner annular member Rs. As with thewindage cover 84 shown in FIG. 1, upon cutting of the rings, the arms196,198 diverge due to the induced stresses in the ring during theforming process and the segments move to the greater radius of curvatureRbu that the segments have in the uninstalled condition.

The method of forming a windage cover discussed above with respect toFIG. 4-12 provides several advantages during assembly and for theproduct which is produced by the method. For example, in the embodimentshown in FIG. 2, joining two rings together, such as the first ring 302and second ring 312, positively locates the rings with respect to eachother and need only be done once prior to forming the segments. If doneafter forming the segments, each segment of the ring must be separatelylocated as each segment of the windage cover is formed. The presentmethod simplifies and facilitates the assembly process, reducing costand material handling time. As discussed above, all rings may bepositively located with respect to each other prior to cutting therings. The segment of the windage cover 84 shown in FIG. 1 has the outerarm 96 positively located with respect to the inner arm 98 on aparticular segment and also each arm is in circumferential alignmentwith respect to adjacent segments. This is also true of the adjacentsegments for the construction of the windage cover 184 shown in FIG. 12.In one particular embodiment, an advantage of the windage cover which isproduced by the process is the circumferential alignment of adjacentseal land segments in the installed condition which are attached to thesegments of the windage cover precursor 184 p prior to cutting theprecursor to form the segments.

Finally, the method of forming the windage covers by bending portions ofthe windage cover precursor 84 p, 184 p which are in the form ofcomplete rings prior to cutting the segments has advantages in providingcoulomb damping as discussed above. In particular, this causes the armsto diverge and the radius of curvature of the segments to increase ascompared to the installed condition. A spring fit is made possible bythe relative dimensions of thickness and length of the base and arms forthe windage covers and provides a damping force as discussed abovewithout further bending of the segments after cutting the windage coverprecursor 84 p, 184 p to form the segments. The divergence of the armsis exaggerated for purposes of illustration in the drawings and istypically less than about five degrees. The increase in the radius ofcurvature is also exaggerated and is typically less than two percent ofthe radius of curvature. Higher values may be warranted depending on aparticular construction and need for damping.

During operation of the rotary machine 10, the stream of working gasespasses through the rotor blades 26, 26 d and through the stator vanes 42along the annular flow path 14. As discussed earlier, the highrotational speed of the rotor disk and blade assemblies 24, 24 d causeregions of high velocity swirling gases to form in the first cavity 56and third cavity 64 adjacent to the second cavity 62. As these gasescontact structure extending into the cavity, they lose energy to theprojections causing wasteful heating of components that extend into thecavity. This energy is replaced by taking energy (that is, a loss ofenergy) from the rotating components. Thus, an advantage is theefficiency of the rotary machine which results from bounding the firstcavity and third cavity with a relatively smooth boundary that isprovided without bolts, for example, protruding into the cavity to holdthe windage cover. In addition, the efficiency of the machine benefitsfrom the reduced volume of swirling gases in the adjacent first cavityand third cavity by providing windage covers at the upstream side 74 anddownstream side 78 of the second cavity adjacent to the edges 70, 70 dof the shroud assembly 44. Reducing the volume of these cavities reducesthe mass of the swirling gases that are pulled along by the rotatingstructure as compared to structures that do not have the windage coversclose to the upstream edge and downstream edge of the shroud assembly.

The structure is relatively lightweight as compared to structures thatare formed of relatively thick windage covers that are bolted in placeand that are heavier than the boltless construction for the windagecover that is shown. Reduced fabrication costs result from using thinstructures that may be formed in a rolling mill as compared to heavierconstructions that must be cast or machined to form the contours of thewindage cover and then drilled to attach the windage cover to the statorstructure.

Another advantage is the durability of the windage cover even though itis relatively lightweight and formed of relatively thin material. Thedurability results from decreasing vibrational stresses in the windagecover by damping vibrations in the windage cover. In one particularembodiment, the damping of vibrations results from using both coulombfriction and viscous friction to damp vibrations in the windage cover.In one detailed embodiment, damping by viscous friction results fromforming the windage cover of relatively thin sheet metal material whichpermits deflections of the walls of the windage cover in response tooperative forces from gases in the cavities adjacent to the windagecover. Thus, the characteristic of its thinness and flexibility thatreduces weight but makes vibrations a concern acts to reduce thosevibrations by providing viscous damping.

An advantage in the detailed embodiments shown in FIG. 1 and FIG. 2 isthe durability of the rotary machine which results from trapping withthe windage cover any structure which might become loose in the secondcavity 62, such as the nut and bolt fasteners 79. As will beappreciated, these items may become liberated during engine operationand enter the flow path causing damage to the rotating structure andstator structure of the machine

Although the invention has been shown and described with respect todetailed embodiments thereof, it should be understood by those skilledin the art that various changes in form and detail may be made withoutdeparting from the spirit and scope of the claimed invention.

1. A method of forming a windage cover for a rotary machine. the rotarymachine having a cavity and at least one damping surface adjacent thecavity, the windage cover for covering at least one side of the cavityin the rotary machine, the windage cover having the form of a ring withat least two circumferentially extending segments that extend about anaxis Am and engage a damping surface, each segment having across-sectional shape which includes a circumferentially extending base,the base having a first side which extends circumferentially about thebase and at least one arm extending from the first side which issubstantially perpendicular to the base, including the steps comprising:forming a continuous ring precursor that extends about an axis Am from acontinuous sheet of annular material, the sheet of annular materialextending about and in a direction perpendicular to the axis Am, thesheet having an inner diameter edge and an outer diameter edge, whereinthe step of forming the continuous ring precursor further includesbending the inner diameter edge and a portion of the continuous sheetadjacent to the inner diameter edge more parallel to the axis Am suchthat the ring precursor has an annular inner flange at the innerdiameter having a radius of curvature with respect to the axis Am andhas a rim which extends from the flange to the outer diameter edge, atleast a portion of the rim extending about and in a directionperpendicular the axis Am, the inner flange being substantiallyperpendicular to the rim and parallel to the axis Am for engaging thedamping surface; cutting the continuous ring precursor to form at leasttwo arcuate segments of the windage cover; wherein stresses induced inthe ring precursor by the process of forming the ring precursor causeeach segment of the windage cover to have an inner flange having agreater radius of curvature with respect to the axis Am than the innerflange of the ring precursor and causes the flange of the segment of thewindage cover to be angled more with respect to the axis Am than theflange of the ring precursor.
 2. The method of forming a windage coverof claim 1 wherein the step of bending the inner diameter edge and aportion of the continuous sheet forms the inner flange of the continuousring precursor which provides an arm for the segments of the windagecover and forms at least a portion of the rim which provides a portionof the base for the segments of the windage cover.
 3. The method offorming a windage cover of claim 2 wherein the windage cover has asecond side which extends circumferentially about the base and which isspaced from the first side leaving the base therebetween and wherein thesteps of the method include prior to cutting the ring, the step ofbending the outer diameter edge and a portion of the ring adjacent tothe outer diameter edge more parallel to the axis Am such that the ringhas an annular outer flange and the rim extends from the inner flange tothe outer flange, the flanges being substantially perpendicular to therim and parallel to the axis Am.
 4. The method of forming a windagecover of claim 2 wherein the rim of the continuous ring precursorprovides a portion of the base for the segments of the windage cover,wherein the arm of the windage cover is a first arm extending from thebase, wherein the step of forming the continuous ring precursor furtherincludes bending the outer diameter edge and a portion of the continuoussheet adjacent to the outer diameter edge more parallel to the axis Amsuch that the continuous ring precursor has an annular outer flange atthe outer diameter having a radius of curvature with respect to the axisAm, the outer flange providing a second arm extending from the base forthe segments of the windage cover, and wherein the first and second armsof the segments of the windage cover diverge more away from each otheras the arms extend from the base than do the inner and outer flanges ofthe continuous ring precursor prior to the step of cutting thecontinuous ring precursor.
 5. The method of forming a windage cover ofclaim 1 wherein the continuous ring precursor is a first ring and theinner flange is a first flange having an axially oriented inner surfaceand wherein prior to cutting the first ring, the method includes thesteps of forming a second ring having a second flange in the samefashion as the first ring, the second flange of the second ring having agreater diameter than the first flange of the first ring; disposing therings about the axis Am such that the second flange of the second ringis spaced radially from the first flange of the first ring leaving anannular channel therebetween; forming a third ring in the same fashionas the first ring, the third ring having a third flange which has anaxially oriented inner surface and a forming a fourth ring in the samefashion as the second ring, the fourth ring having a fourth flange whichare oriented to face the first and second flanges on the first andsecond rings in the installed condition; disposing the third and fourthrings about the axis Am such that all rings have the same axis Am andthe third and fourth flanges form an annular channel for the third andfourth rings that axially faces the annular channel of the first andsecond rings; disposing a circumferentially continuous base member aboutthe axis Am which engages the inner surface of the first flange of thefirst ring and the inner surface of the third flange of the third ring;attaching the first and second rings together; attaching the third andfourth rings together; attaching the base member to the inner flange ofthe first ring and the inner flange of the third ring to form the base;cutting the first, second, third and fourth rings and the base along aplane containing the axis Am.
 6. The method of forming a windage coverof claim 5 wherein the step of attaching the rings and base membersincludes brazing the rings and base members.